My uprated brake system worked well up until last week. After installing braided stainless flex lines (in the front only) and bleeding the system with a power-bleeder setup I could not get the car to brake properly.

This is a 140/120(69-70) setup boosted split circuit (front/rear circuits). As I said it has worked quite well for nearly a year.

The loss of brakes was pretty bad. Only the bottom end of the pedal and a quick pump would stop the car. The feel was a bit "spongy" and reminded me of way too much air in the line (as though any is okay, right?)

So, I bled the system again, and again, completely flushing out the old fluid. Still no improvement.

My verdict was a failed master cylinder. I ordered a rebuilt replacement and it finally arrived today. A perfect fit. I found leaked brake fluid on the backside of the 140 booster, so I was pretty sure I found the problem...

Knowing I'd have to bleed the whole system again, I decided to go ahead and replace my rear brake shoes and install the rear stainless braided flex line. I also lubed up my rear axle bearings while I was in there.

I finished all that and after a thorough bleed found that brake situation unchanged. WTF?! I'm not sure if the MC was really the problem now.

I've checked and re-checked the brake lines at all the junctions and found no leaks. I've bled the system again, I've test driven the car, I've found no change at all. The only variable I can see are the stainless lines, but
they came directly from a working donor car and after being cleaned and drained, went right onto the 68. I put a little anti-seize on the fittings, but kept that off actual connections - plus, I've checked several times for leaks and found none.

I'm frustrated and stumped. Brakes are a pretty simple but critical system.
I must be overlooking something - but what?

Any advice or comments are appreciated.